1. Field of the Invention
The present invention relates to a method suitable for use in manufacturing a coin-type or button-type alkaline battery used in small-sized electronic equipment such as an electronic wrist watch, an electronic desk calculator, or the like.
2. Description of the Related Art
Conventionally, as an alkaline battery of a coin type or button type used in an electronic wrist watch, electronic desk calculator, or other small-sized electronic equipment, there has been proposed the one shown in FIG. 3. With reference to FIG. 3, an example of a button-type alkaline battery will be explained.
In FIG. 3, numeral 1 denotes positive electrode mixture containing silver oxide or manganese dioxide as a positive active material, and in this example the positive electrode mixture 1 is formed into a pellet of a coin shape. The positive electrode mixture of the coin-shaped pellet is provided in a positive electrode can 2, which is composed of a nickel-plated stainless steel plate and functions as a positive electrode terminal and a positive electrode current collector.
Also, numeral 3 denotes negative electrode mixture in gel state without mercury, containing zinc or zinc alloy powder as a negative active material and composed of alkaline electrolyte such as sodium hydroxide aqueous solution or potassium hydroxide aqueous solution, thickener, and the like; and the negative electrode mixture 3 is provided in a negative electrode cup 4 functioning as a negative electrode terminal and a negative electrode current collector.
Between the positive electrode mixture 1 and the negative electrode mixture 3, a separator 5 composed of a graft-polymerized three-layer film consisting of non woven fabric, cellophane, and polyethylene is provided. Then, the separator 5 is impregnated with alkaline electrolyte, for example, sodium hydroxide aqueous solution or potassium hydroxide aqueous solution.
A gasket 6 made of nylon is provided on the inner periphery of the positive electrode can 2 between the upper surface of the separator 5, and a cuff portion 4a and a cuff bottom portion 4b of the negative electrode cup 4; and the positive electrode can 2 is swaged and caulked to be hermetically sealed.
As shown in FIG. 4, the negative electrode cup 4 in this example is formed of a three-layer clad member made of nickel (Ni) 7, stainless 8, and copper (Cu) 9; and on the periphery thereof the cuff portion 4a and cuff bottom portion 4b are provided. Then, the inside surface area of copper 9 other than the cuff portion 4a and cuff bottom portion 4b is coated with stannum (Sn) having a higher hydrogen overpotential than copper to provide a stannum coating layer 10.
There has been known such an example of Patent document 1, in which only on the inside surface area of copper 9 other than the cuff portion 4a and cuff bottom portion 4b is coated with stannum (Sn) to provide the stannum coating layer 10. In Patent document 1, the art relating to a coin type and button type alkaline battery is described in which the inside surface of the negative electrode cup 4 other than the cuff portion 4a and cuff bottom portion 4b thereof is coated with metal or metal alloy having a higher hydrogen overpotential than copper by means of a dry-type film forming method.
In the above alkaline battery, hydrogen gas (H2) can be prevented from being generated, and since metal or metal alloy without using mercury having a higher hydrogen overpotential than copper is not deposited on the cuff portion 4a and cuff bottom portion 4b of the negative electrode cup 4, rising-up of the alkaline electrolyte (creep phenomenon) can be prevented and liquid leakage resistance is not deteriorated; and further since metal or metal alloy having a higher hydrogen overpotential than copper is deposited by means of a dry-type film forming method, the cuff portion 4a and cuff bottom portion 4b of the copper surface 9 are not oxidized, so that the liquid leakage resistance can be maintained against the creep phenomenon of the alkaline electrolyte.
Conventionally, when the inside surface area other than the cuff portion 4a and cuff bottom portion 4b made of copper 9 of the negative electrode cup 4 is coated with stannum (Sn) having a higher hydrogen overpotential than copper to provide the stannum coated layer 10, sputtering has been used. A mask used for sputtering in Patent document 1 is, for example, shown in FIG. 1, in which a circular counter-boring portion of 6 mm in diameter where the negative electrode cup 4 is mounted and fastened with a screw and a circular opening of 5 mm in diameter provided at the center of the counter-boring portion causing sputtering atoms to reach the inside of the negative electrode cup 4 are provided; and a negative electrode cup mounting portion 21C is formed on the negative electrode cup mounting side of the mask 21. The negative electrode cup 4 is set such that the negative electrode cup mounting portion 21C is covered by the negative electrode cup 4 put from the negative electrode cup supplying side (from the upper side in the figure), and then sputtering atoms are projected from the negative electrode cup mounting side (from the lower side in the figure) to be deposited. The mask is made of comparatively thin metal plate or the like and is shaped having the negative electrode cup mounting portion 21C as illustrated.
[Patent Document 1]
Japanese Published Patent Application 2002-198014 (page 2, FIG. 3)
A conventional mask 21 proposed in the above mentioned Patent document 1 and used in sputtering, when partly depositing film such that the inside surface area other than the cuff portion 4a and cuff bottom portion 4b of the negative electrode cup 4 is coated, has a required opening provided in a comparatively thin metal plate or the like to deposit a film on a predetermined portion of the inside surface of the negative electrode cup 4. However, the mask 21 is deformed by heat generated in sputtering or the like to cause position dispersion with respect to the negative electrode cup 4 mounted on the mask 21, so that the stannum coating layer 10 can not be favorably formed on the negative electrode cup 4, which results in inconvenience when mass production of the negative electrode cup 4 is conducted.
Further, such deformation generates gap between the negative electrode cup 4 and the negative electrode cup mounting portion 21C, so that there occurs possibility of depositing metal or metal alloy having a higher hydrogen overpotential than copper on the cuff bottom portion 4b or other areas; and also there is a problem of a shorter operating life due to the thin plate thickness of the mask, in which such deformation as accompanied by elimination and cleaning of an excessively-coated film adhered to the mask 21 occurs, because necessary chamber maintenance is periodically performed for stabilizing the quality of film in a film depositing apparatus at mass production.
In light of the above, the present invention provides a method for manufacturing an environmentally-benign alkaline battery without containing mercury (Hg), in which a mask used when PVD (physical vapor deposition) method of a dry-type film forming method is employed is appropriately shaped and the inconvenience at mass production of the negative electrode cup is improved.